Abstract

Global navigation satellite systems-reflectometry
(GNSS-R) is an emerging technique that uses navigation opportunistic
signals as a multistatic radar. Most GNSS systems operate
at L-band, which is affected by the ionosphere. At present, there
is only a GNSS-R space-borne scatterometer on board the UK
TechDemoSat-1, but in late 2016, NASA will launch the CYGNSS
constellation, and in 2019, ESA will carry out the GEROS experiment
on board the International Space Station. In GNSS-R,
reflected signals are typically processed in open loop using a short
coherent integration time (~1 ms), followed by long incoherent
averaging (~1000 times, ~1 s) to increase the signal-to-noise ratio.
In this study, the global ionospheric scintillation model is first
used to evaluate the total electron content and the scintillation index
S4 . The ionospheric scintillation impact is then evaluated as
a degradation of the signal-to-noise ratio, which can be used to
assess the altimetry and scatterometry performance degradation
in a generic GNSS-R mission. Since ionospheric scintillations are
mostly produced by a layer of electron density irregularities at
~350 km height, underneath most LEO satellites, but closer to
them than to the Earth’s surface, intensity scintillations occur especially
in theGNSS transmitter-to-ground transect, therefore, the
impact is very similar in conventional and interferometric GNSS-R.
Using UK TechDemoSat-1 data, signal-to-noise ratio fluctuations
are computed and geo-located, finding that they occur in the open
ocean along ~±20° from the geomagnetic equator where S4 exhibits
a maximum, and in low wind speed regions, where reflected
signals contain a non-negligible coherent component.